Gas furnaces for melting aluminum: selection criteria

Reflective gas and shaft furnaces are widely used in most aluminum foundries, as in the production of shaped casting, i.e. cast aluminum parts and products, and in the production of aluminum ingots, which are then applied in rolling, extrusion or forging of aluminum, as well as melting ingots for other foundries.

The basic types of gas reflective furnaces for melting aluminum are [1]:

  • Reflective dry hearth melting furnaces (dry hearth reverberatory furnaces), in which the metal before, how to melt, heated on the inlet inclined hearth (figure 1).
  • Reflective Melting Furnaces bathroom type (wet bath reverberatory furnaces), in which the metal is loaded directly into the aluminum melt, usually without preheating (figure 2).
  • Shaft smelting furnaces (stack (shaft) furnaces) are characterized by increased energy efficiency, since they use the heat of waste gases to heat the loaded aluminum charge (figure 3). Shaft furnaces are modified version of reverberatory furnaces

Figure 1 - Reflective gas-fired dry hearth melting furnace [2]

Figure 2 - Reflective gas tank type melting furnace [3]

Figure 3 - Gas shaft melting furnace [4]

Another type of gas furnaces are crucible furnaces., which are used for small volumes of casting, as well as for special alloys. These ovens require separate consideration..

Additional functions of gas ovens

Each of these three basic oven types can have additional options., as well as elements, typical for other types of ovens, eg:

  • all three types of furnaces can have external wells for charging crushed charge into the melt, for example, shavings;
  • inclined hearth furnaces and shaft furnaces can have additional charging ports for charging the charge directly into the melt;
  • bath furnaces and shaft furnaces can have a wide threshold or a short inclined hearth at the entrance to the furnace for preheating ingots and massive, for example, packaged scrap, and also for the convenience of removing slag from the surface of the melt;
  • all types of furnaces can have a melt stirring system, as in the loading well, so in a bath of aluminum melt.

Stationary and tilting ovens

Each of the three basic types of melting furnaces can be made in two versions:

  • Stationary ovens: cheaper and less convenient for dispensing metal, can have a capacity of up to 100 tons and higher melting capacity 10 tons per hour.
  • Tilting Furnaces: More expensive and more convenient for dispensing or transferring finished molten metal.

Reflective dry hearth melting furnaces

In dry hearth furnaces, the charged aluminum charge is placed on an inclined hearth, which is above the level of the molten metal (figure 4). Hot exhaust gases pass through the charge and quickly heat it up. The charge melts and gradually flows from the inclined hearth into the chamber for the accumulation of molten metal, which is therefore often called a piggy bank.


Figure 4 - Reflective dry hearth melting furnace.
Heated ingots are visible at the entrance to the furnace [5]

Dry hearth furnaces are commonly used, when:

  • Most of the charged aluminum charge is smelting ingots, including, large, that is, shita elements have a minimum surface.
  • Aluminum charge contains, for example, steel elements, as well as the remains of casting molds, for example, sandy. The aluminum part of the charge is melted on the inclined hearth and the aluminum flows down the inclined hearth, and foreign materials remain on the hearth and are removed from the oven using scrapers.
  • Aluminum batch may contain water or be wet.

Advantages of dry hearth furnaces:

  • Security. The entire charge is melted on an inclined hearth, not below the surface of the melt. Such heating eliminates the ingress of water into the melt and the threat of explosion..
  • Prevention of melt contamination. Allows you to remove extraneous from the charge, not aluminum, materials, which remain on the inclined hearth.
  • Ability to produce large volumes of liquid aluminum almost continuously.

Disadvantages of dry hearth ovens:

  • Usually these stoves have two chimneys, one above the inclined hearth and the other above the piggy bank. This requires a burner control system for optimal operation [1].
  • High waste of aluminum (7-12 %) when melted in a furnace atmosphere at a temperature 700-760 degrees Celcius. Waste is the loss of metallic aluminum, which was contained in the loaded charge, as a result of its oxidation and transition to slag.
  • Low energy efficiency. The efficiency of this type of furnace increases with an increase in the ratio of the volume of the charged charge to its outer surface.. Maximum efficiency is achieved when melting large aluminum ingots, sometimes weighing up to 500 kg [1].

Reflective Bath Type Melting Furnaces

In these furnaces, the combustion products are in direct contact with the surface of the melt and heat transfer occurs as a result of a combination of convection and radiation.. These ovens have a lower roof to maximize the use of radiant heat, the most efficient way to transfer heat to aluminum. The heat source (burners) can be located at a distance 45-60 mm from the mirror of the aluminum melt. The closer the heat source is to the surface of the melt, the faster heat is transferred to it [1].

Figure 5 - Reflective bath type melting furnace [6]

Bath type reflective melting furnaces are used, when:

  • Aluminum charge consists mainly of one alloy or related alloys, and the change of alloys is rare.
  • The charge can have a large surface, and its components have different sizes, including, small thickness, for example, scrap aluminum profiles, as well as packaged aluminum scrap.
  • Large volumes of molten aluminum required.

Advantages of Reflective Bath Type Ovens:

  • The installation cost of such furnaces is the lowest compared to dry hearth and shaft furnaces.
  • All metal melts under the surface of the aluminum melt, which provides a relatively low waste of metallic aluminum 2-5 %.
  • Allows various additions depending on the conditions of specific foundries: a small under or threshold for preheating ingots or stacked scrap, loading well for crushed scrap (figure 5), melt mixing, window for easy degassing, etc.. P.

Disadvantages of bathroom type ovens:

  • Have an increased ratio of furnace capacity for molten metal to furnace melting rate, which requires additional production space. A typical such relationship is 8 to 1.
  • If smelting ingots are used, then they require preheating on the threshold or hearth before being loaded into the melt.
  • The lower roof of the oven requires extra care when cleaning.

Shaft smelting furnaces

Shaft smelting furnaces have a higher energy efficiency due to better sealing of the furnace from atmospheric air penetration and the use of exhaust gases for preheating the aluminum charge.

The aluminum charge passes down the feed shaft and gradually reaches the melting zone, where it melts under the influence of the burners and flows down into the piggy bank. Hot exhaust gases pass from the melting zone through the mine and heat up the charge..


Figure 6 - Shaft melting furnace [7]

Benefits of shaft furnaces

  • Due to the preliminary heating of the charge with the waste products of combustion, the energy efficiency reaches 40-50 %.
  • The finished aluminum melt is characterized by a reduced content of hydrogen and non-metallic inclusions. This is achieved by separating the melting zone and the preheating zone.. Moisture and other contaminants evaporate or burn out while still in the mine, and a dry charge with a minimum of polluting materials enters the melting zone.
  • No excessive contact of the melt with atmospheric oxygen, which provides a reduced level of waste of metallic aluminum (0.8-1.5 %).

Disadvantages of shaft furnaces

  • For, so that shaft furnaces achieve low waste and high energy efficiency, they must be operated strictly in accordance with the rules established for them, including the use of a sufficiently dense aluminum charge. With a lightweight and thin-walled charge, shaft furnaces can have an increased waste of aluminum and a reduced energy efficiency..
  • Shaft furnaces have a fairly large overall height (about 6 m), which is not suitable for all industrial premises.
  • Refractory lining at the bottom of the shaft furnace charging well is exposed to increased mechanical shock loads, which leads to the need for more frequent maintenance.
  • Shaft furnaces are more difficult to operate, than reflective and therefore require additional qualified personnel.

Total Cost of Ownership

The concept of “Total Cost of Ownership” (TCO) is currently used for the optimal selection of the furnace for melting aluminum [8]. This approach is to calculate the sum of all expenses, which are associated with the acquisition, setting, operation and maintenance of the equipment in question. The result is the full cost of producing one ton of finished molten aluminum, taking into account all costs over the life of the furnace..

This total cost of finished molten aluminum includes:

  • Capital costs for the purchase and installation of the furnace itself, as well as related equipment:
    – Bake
    – Loading system for aluminum charge
    – Aluminum melt discharge system
    – Instruments and instruments for furnace control and melt quality control

  • Operating costs for furnace operation and molten aluminum production:
    – Energy resources: gas, electricity, liquid fuel. The energy efficiency depends on the type of oven (inclined hearth, bathroom type, mine) and rated melting rate.
    – Labor costs: wages of workers and managers, including, maintenance personnel. Depends, including, on the complexity of the equipment, frequency of its maintenance, the required level of personnel qualifications.
    – Materials: cost of materials, which are consumed during the operation of the furnace, for example, the cost of the aluminum charge; amount of aluminum, which is lost through oxidation when melted in a furnace, and also remains in the slag; fluxes, etc.. P.

The application of this approach to the analysis of reflective and shaft smelting furnaces led to rather unexpected conclusions [8]:

  • When deciding to install a new melting furnace, it is necessary to consider not only its purchase price., then also the full cost of smelting one ton of metal, taking into account all the past, present and future expenses (TCO).
  • established, that the capital costs for the purchase of the furnace can be in the full cost of a ton of metal only 3-5 %.
  • Total cost of aluminum, smelted in shaft and reverberatory furnaces differs slightly [7].
  • Melting technology, which is used in the oven, greatly affects the cost of materials and energy resources. Recognized, that shaft furnaces are currently the most efficient gas melting equipment with high thermal efficiency (40-48 %) and low waste (1-2%).
  • Despite, that shaft furnaces, It is believed, have limited technological flexibility, they are the main melting equipment of any medium-sized shaped casting company.
  • On the other hand, is, what is the cost of labor, which is necessary for the operation of shaft furnaces, about twice as high, than for reflective ovens. This is due to the higher complexity of shaft furnaces., which requires the involvement of additional specialists to manage and maintain them.
  • In the same time, Cost of materials, which consume reflective ovens is significantly higher, than shaft furnaces, mainly due to higher losses of aluminum during melting in reverberatory furnaces, as well as their low energy efficiency.

Sources:

  1. ASM Handbook – Volume 15 – Casting – 2008.
  2. https://dynamofurnaces.com
  3. Aluminium technologies / Dr. YUCEL BIROL – Dokuz Eylul University, Turkey – 29.09.2015
  4. https://www.strikowestofen.com
  5. https://www.bjholland-equip.com
  6. https://www.andritz.com
  7. http://www.marconiforni.com
  8. Investment and Management Decisions in Aluminium Melting: A Total Cost of Ownership Model and Practical Applications / A. Bacchetti, S. Bonetti, M. Perona and N. Saccani – Sustainability 2018, 10, 3342